The present invention relates to composite glass articles suitable for the manufacture of glass channel plates and the like. Channel plates, bundles and other channeled bodies are useful for fabricating such products as electronic channel amplifiers, diffusion cells for gas purification, image transfer devices and other articles requiring intricate arrays of holes in glass difficult to obtain by mechanical machining methods.
One method for making channel plates or bundles involves sealing solid rods of leachable core glass within tubular pieces of non-leachable skin glass. The resulting glass subassemblies are then drawn, stacked, fused, redrawn, etc. to ultimately provide a composite glass article consisting of an interconnected glass matrix composed of the skin glass which encases an array of leachable glass core elements. The ends of these core elements are exposed and the elements are finally removed with a leaching medium such as nitric acid, leaving a glass body comprising an array of channels consisting of the interconnected skin glass matrix.
One such method for fabricating a channeled body is described in U.S. Pat. No. 3,294,504 to J. W. Hicks, Jr. Nitric acid-soluble glasses specified in that patent for use as leachable core elements or the like include two barium borosilicate glasses comprising substantial quantities of lanthanum and thorium oxides. The skin or matrix glass is a relatively durable alkali borosilicate glass.
Another method for producing a channeled body is described in U.S. Pat. No. 3,502,455 to W. L. Gardner. Again, a barium borosilicate glass comprising lanthanum and thorium is suggested for use as a leachable core glass, while a durable alkali borosilicate glass is used to provide the matrix.
Properties of major commercial importance for composites to be used in channel plate manufacture are the dissolution rate of the core glass and the difference between that rate and the dissolution rate of the matrix glass. Core glasses which dissolve comparatively rapidly in conventional leaching media offer a significant economic advantage in terms of processing time. A substantial difference between the dissolution rates of the core and matrix glasses is required, particularly where long core elements are to be removed, since any significant dissolution of the matrix glass produces channel tapering which is unacceptable for many applications.
It is also important that the coefficient of thermal expansion of the core glass be reasonably closely matched to that of the matrix glass so that excessive thermal stresses in the composite article may be avoided. Although the core glass is ultimately removed from the composite, excessive stress therein prior to core removal can produce fractures or make cutting of the composite impossible.
The process of fabricating composite articles for this use comprises fusing and drawing steps during which the core glasses are fused into the skin glasses, drawn with the skin glasses to reduce the cross-sectional area of the cores, and the core and skin combinations thereafter fused into an integral composite. It is desirable that the core glass have a softening point in the range of about 50.degree.-100.degree. C. higher than that of the skin glass in order to minimize distortion of the shape of the cores by the skin glass during these steps.
Finally, the core glass must be one which can be essentially entirely leached from the matrix glass without leaving harmful insoluble residues. The removal of such residues, particularly from very small channels, is both difficult and costly.
It is a principal object of the present invention to provide glass compositions for leachable core elements which meet the necessary thermal expansion and softening point requirements for composite manufacture, while at the same time offering improved leaching behavior to facilitate core removal.
It is a further object of the invention to provide glass compositions for leachable core elements at reduced cost as compared with glasses containing La.sub.2 O.sub.3 and ThO.sub.2.
It is a further object of the invention to provide composite glass articles suitable for the manufacture of channeled bodies which comprise glass core elements exhibiting improved leaching behavior.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.